In recent years, liquid crystal display elements are expected not only in the conventional application to personal computer monitors but also in application to ordinary color televisions. The color gamut of color liquid crystal display elements is determined by chromaticity of light emitted from red, green and blue pixels and, where chromaticity points of the respective color pixels in CIE system of color representation are represented by (xR,yR), (xG,yG), (xB,xB), the color gamut is defined by an area of a triangle surrounded by these three points on a x-y chromaticity diagram. Namely, the larger the area of this triangle, the more vivid color image the display elements reproduce. The area of this triangle is usually expressed using a ratio of the area of the triangle to the area of a reference triangle formed by three points of the three primary colors, red (0.67,0.33), green (0.21,0.71) and blue (0.14,0.08), in the standard system defined by US National Television System Committee (NTSC) (in unit of %, hereinafter referred to simply as “NTSC percentage”). Ordinary notebook computers have the values of approximately 40 to 50%, desktop computer monitors the values of 50 to 60%, and existing liquid crystal TVs the values of approximately 70%.
A color image display device utilizing such color liquid display elements is mainly composed of light shutters utilizing liquid crystal, a color filter having red, green and blue pixels, and a backlight for transmission illumination, and the chromaticity of light emitted from the red, green and blue pixels are determined by the emission wavelengths of the backlight and the transmittance spectrum of the color filter.
In the color liquid crystal display elements, the color filter extracts only wavelengths in necessary regions from the emission distribution of the backlight to provide the red, green and blue pixels.
Methods for production of this color filter proposed heretofore include such methods as dying, pigment dispersion, electro-deposition, printing, inkjet, and so on. The colorant for coloring used to be dyes, but are now pigments in terms of reliability and durability as liquid crystal display elements. Accordingly, the pigment dispersion is most commonly used as a method for production of the color filter at present in terms of productivity and performance. In general, in use of an identical colorant, the NTSC percentage and brightness are in a trade-off relation and are appropriately used according to applications.
On the other hand, the backlight generally used is one using as a light source a cold cathode fluorescent lamp with emission wavelengths in the red, green and blue wavelength regions and using a light guide plate for converting a light emitted from this cold cathode fluorescent lamp into white area light source. In recent years, a LED is becoming used from such a viewpoint that it has a long life, it requires no inverter, it provides a high brightness and it is free from mercury.
A conventional LED backlight uses blue light emitted from a LED and a yellow phosphor obtained by excitation using the blue light as a white area light source.
However, the above-described light source employs a yellow phosphor, and thus there are a great deal of emission of lights, with unnecessary wavelengths in terms of the color purities of red and green, and it has been difficult to obtain a high gamut display. As a countermeasure, it is possible to cut out lights with unnecessary wavelengths by a color filter to increase the color purities of red and green in principle, but most part of emission from the backlight is cut out, and thus the brightness will significantly decrease. Particularly by this method, red emission will significantly decrease, and accordingly it is practically impossible to reproduce a strongly reddish color. Further, the emission wavelength distribution of a LED has such characteristics that the emission intensity is high in a range of from 460 to 480 nm as compared with that of a cold cathode fluorescent lamp. The light in this wavelength region deteriorates the color purity of a blue pixel and thereby is required to be cut out by a blue color filter as far as possible. However, a conventional blue composition for a color filter (hereinafter sometimes referred to simply as a blue composition or a blue resist) and a color filter have not had sufficient performance to cut out lights with wavelengths of from 460 to 480 nm, and are not sufficiently suitable for a color image display device using a LED as a backlight.
In order to overcome this problem, a method of combining a LED which emits red, green and blue lights has been proposed (Non Patent Document 1) in recent years, and a display having an extremely high gamut has been produced experimentally by this method. However, since red, green and blue LED chips are independently assembled, there are such problems that 1) mounting takes effort, 2) red, green and blue LED chips are disposed with limited distances, and thus the distance from the respective LED chips to a light guide plate is required to be long for sufficient color mixture of emission from the respective LED chips, and 3) integral multiple numbers of the respective LED chips are combined to adjust the white chromaticity, and thus the white balance cannot continuously be adjusted.
Non Patent Document 1: Monthly DISPLAY April 2003, p. 42 to 46